1. Field of the Invention
This invention relates to glass compositions suitable for strengthening by ion exchange and particularly relates to the strengthening of SiO.sub.2 --Na.sub.2 O--Al.sub.2 O.sub.3 --ZrO.sub.2 glasses by potassium ion exchange.
2. Brief description of the Prior Art
U.S. Pat. Nos. 3,485,702 and 3,752,729, both to Mochel, deal with an improved glass composition of the SiO.sub.2 --ZrO.sub.2 --Al.sub.2 O.sub.3 --alkali metal oxide systems for chemical strengthening. These references disclose that the incorporation of 5 to 25 percent, preferably 10 to 25 percent by weight of ZrO.sub.2 in the glass composition results in deep ion exchange penetrations with resultantly high compressive stresses in relatively short periods of time. Unfortunately, these high ZrO.sub.2 containing glasses are not compatible with conventional float and updraw forming methods because of high melting and forming temperatures and unfavorable liquidus temperature-viscosity relationships. U.S. Pat. No. 3,772,135 to Hara et al. discloses glass compositions for chemical strengthening that represent some improvement in temperature-viscosity relationships, but still exhibit undesirably high melting temperatures, and disadvantageously limit the ZrO.sub.2 content to low levels or eliminate it completely.
The liquidus is defined as the temperature at which devitrification or uncontrolled crystallization of the glass first appears as the temperature is lowered. At temperatures slightly below the liquidus, devitrification occurs, sometimes at a relatively rapid rate, and if uncontrolled could ruin the flat glass sheets, severely curtailing production yields. Somewhat related to liquidus and a problem of devitrificaton is the working range of the glass. The working range is defined for the purposes of this invention as the temperature interval between the glass-forming temperature which is usually taken as the glass temperature when the log of the viscosity of the glass is equal to 4 and the liquidus temperature. Melting and forming temperatures are defined for the purposes of this invention as the temperature at which the viscosity of the glass is equal to about 100 poises and 10,000 poises, respectively.
In flat glass manufacturing, it is desirable for a glass to have low melting and forming temperatures to enable easy working of the glass, to conserve fuel and to prevent excessive thermal deterioration of the glass melting and forming equipment. Further, in flat glass manufacturing, particularly by the float and the updraw process, it is desirable that the glass have a low liquidus temperature and a wide working range. A low liquidus temperature insures against devitrification in cold spots of the furnace and a wide working range insures against devitrification. In the glass forming area of the furnace. In the case of manufacturing glass by the updraw process such as the Pittsburgh Process by liquidus temperature of the glass is about 1840.degree. F. and the working range of the glass is about 70.degree. F. Commercial float glass has a liquidus temperature of about 1830.degree. F. and a working range of about 45.degree. F.
ZrO.sub.2 has a pronounced effect on the liquidus temperature and the working range in the family of glasses disclosed in the above-mentioned Mochel patents. With ZrO.sub.2 concentrations above about 51/4 percent, liquidus temperatures begin to increase quite rapidly with increasing ZrO.sub.2. With ZrO.sub.2 concentrations above 51/2 percent by weight, a serious problem develops with respect to float or updraw forming. With these particular glasses, a narrow working range, and in many instances a negative working range, is established, that is, the liquidus temperature is higher than glass-forming temperature and devitrification occurs rather quickly. This could have disastrous effects if such glasses were formed on a commercial scale by the updraw or float methods. In addition, many of the glasses disclosed in the above-mentioned patents have high melting and forming temperatures making them undesirable for commercial flat glass manufacturing.
It is apparent from the above that it would be desirable to provide a family of glass compositions of the SiO.sub.2 --Na.sub.2 O--AL.sub.2 O.sub.3 --ZrO.sub.2 system for use in chemical strengthening which would be more suitable for forming by the updraw and float processes than those SiO.sub.2 --Na.sub.2 O--Al.sub.2 O.sub.3 --ZrO.sub.2 systems disclosed in the prior art. More particularly, it would be desirable to provide a family of glass compositions for ion exchange which have lower melting and forming temperatures and a wider working range than those glass compositions of the SiO.sub.2 --Na.sub.2 O--Al.sub.2 O.sub.3 --ZrO.sub.2 systems disclosed in the prior art for ion exchange.
Besides the references mentioned above, other relevant prior art consists of U.S. Pat. Nos. 3,790,430 to Mochel; 3,498,773 to Grubb et al.; 3,524,737 to Doyle et al.; 3,433,611 to Saunders et al.; 3,416,936 to Sproul, Jr.; 2,252,466 to Hanlein; 3,499,776 to Baak et al.; 2,877,124 to Welsch; 2,978,341 to Bastian et al.; 3,357,876 to Rinehart and British Pat. No. 1,115,972.